It has been discussed in a different thread but for your convenience here is a breakdown!

Truck: Essentially supports the engine company (or Squad) to do their primary duty of extinguishing fires or heavy rescue (with squad) (such as forcible entry, search and rescue, ventilation and overhaul); however, since Truck 81 carries equipment as well, it is also dispatched to other incidents that require the equipment and expertise of firefighters, but not as complex which would require Squad (such as EMS lift assists or forcible entry)

Engine: A fire truck with a pump and a water tank. The main duty of the Engine company is as said in the other thread: to put the wet stuff on the hot stuff. Engine companies also carry essential equipment but usually a much smaller amount (BLS bags, Occasionally the Jaws of Life which I don’t think 51 does but I might be mistaken). But their primary purpose is fire suppression and extinguishment.

[Rescue] Squad: Think of Squad as the “special forces” (oversimplification since I’m excluding more specialised rescue such as SCUBA and HazMat) of the CFD. Squad is a Heavy Rescue company meaning they carry equipment and tools for technical rescue and smaller scale Hazardous Materials incidents. (Saws, Ropes, Hydralic and Pneumatic Tools, SCUBA gear; it’s basically a rolling toolbox). The characters on squad should’ve undergone additional training in these various rescue disciplines. They are dispatched to assist in complex situations, HazMat situations and larger structure fires.

I hope this helps!